The present invention relates to a process of manufacturing semiconductor devices, and more particularly to the method of manufacturing an isolated PN junction of a semiconductor mesa diode.
The manufacture of diodes by employing the conventional planar process, especially the contacting and encapsulating, is entailed by difficulties which are due to the PN-junction positioned on the surface. This PN-junction is protected by an SiO.sub.2 -layer, but owing to pressures exerted during the encapsulation, cracks are likely to result in the SiO.sub.2 -layer so that reverse currents are likely to develop.
As is known, a considerable disadvantage of the planar process is seen in the forming of bent diffusion fronts at the masking edges. It is known that this is not the case with the mesa process, but instead the mesa process is known to have other disadvantages, such as that the devices produced thereby have a non-protected PN-junction, and that the passivation is entailed by difficulties in the case of genuine mesa devices.
The German published patent application (DT-OS) No. 24 42 398 discloses a process of manufacturing a plurality of mesa semiconductor devices in which one side of a semiconductor substrate is provided on its entire surface with a PN-junction area which then, by employing a photolithographic etching process, is divided into mesas in accordance with the number of semiconductor devices, whereupon the substrate surface containing the mesas is provided with an oxidation layer. In this conventional process, the substrate is first readily diffused and the covering masking is applied after the mesas have been etched out. The position of the PN-junction and its angle in relation to the mesa surface is determined in this process, above all by the etching depth, with it being naturally difficult to reach the ideal value of 90.degree. for the angle.
Considering that the masking is effected by thermal oxidation, the diffusion-out at the PN-junction lying at the edge of the mesas may cause a surface depletion at the marginal area. In consequence of this, it may happen that the angle already deviating from the ideal value is still changed more strongly. This increases the danger of a break-down of the loading of the diode.